The present invention relates to power amplifiers e.g., audio amplifiers, operating over a wide range of voltage frequencies and amplitudes handled and driving a varying effective load impedance (e.g., a speaker).
A power amplifier for music reproduction in loudspeakers must reproduce the input voltage amplified by a suitable amount typically in the 20 to 40 db range and supply it to a load which can be resistive or reactive or any combination of the two and of value anywhere from abt 2 ohms to 10 ohms nominal or average, but which can vary between 1 ohm and 100 ohms over the audio range and which increases with temperature and varies instantaneously with level. This voltage varies between 0 and clipping level and typically has a peak to average ratio of between 5 and 20 db.
One important problem with this is that a transformerless output stage with a fixed supply voltage designed for one particular load impedance will be extremely inefficient operating into a much lower impedance and will on the other hand deliver only a fraction of its normal power into a much higher impedance.
Varying the supply voltage in proportion to the square root of the load impedance solves this problem, but is not an optimum solution.
It is an important object of the invention to provide an improvement in power amplifier power supplier with voltage varying means constructed such that at very low and very high average power the available safe maximum output will be much higher than for an amplifier with a fixed supply voltage.
It is a further object of the invention to provide that when the signal processed by the amplifier is music or the like irregular wave form and the load is a speaker or the like, the amplifier will for a majority of operating time operate at a higher voltage hence having a much higher available peak output level compared to levels used for regular wave forms and simple resistive loads, consistent with one or more of the preceding objects.
It is a further object of the invention to provide that the amplifier will give essentially the same power into any impedance from 2 ohms to 8 and the supply will adjust itself (pre set its maximum voltage) for constant worst case dissipation, consistent with one or more of the preceding objects.
It is a still further object of the invention to provide that the output stage temperature will remain within a smaller range since the thermal feedback tends to stabilize the dissipation, hence the temperature, so that long-term reliability will be improved, consistent with one or more of the preceding objects.
It is a still further object of the invention to allow making a variable supply an efficient, simple and inexpensive low-frequency thyristor switching supply, so that losses are substantially limited to the IR drop in the transformer line (power end) of the supplier, consistent with one or more of the preceding objects.
It is a still further object of the invention to make an amplifier with a given supply and output power stage behave as though it had at least twice the power-rating of a conventional amplifier with the same supply and output stage, consistent with one or more of the preceding objects.